1. Field of the Invention
The present invention generally relates to computer keypads. More particularly, the present invention relates to keypad scanners.
2. Description of the Related Art
A wide variety of devices may be used to interface with computer systems. One of the more popular of such devices is a keypad. Computer systems distinguish between keys on a keypad by assigning each key an individual value. These values are typically represented by a row signal and a column signal. When a key is pressed, the keypad presents the value of that key for input into the computer. In processing keypad input, computer systems typically first discern when a key has been pressed for input, and then they register the value of the key that has been pressed. Prior computer systems have used a keypad scanner to discern when a key has been pressed for input into the computer system. The keypad scanner periodically samples the key value presented by the keypad for input into the computer. A clock is used to determine the periodic rate at which the scanner samples the key values. After each sample, the scanner places the key value into a new value latch. The latched new value is then compared to an old value in a second latch. If there is a difference between the two latched values, then a status change signal is generated to indicate to the computer that a key has been pressed for input. Upon receiving a status change signal, the computer system registers the value of the key pressed and the scanner updates the old value latch with the value of the pressed key.
One disadvantage of prior art keypad scanners is the large amount of power required to operate them. Components of a keypad scanner include a clock and two latches, which require a constant supply of power. Also, power must be supplied to circuitry which compares the old latch value with the new latch value. The circuitry must be supplied with power for each cycle of the clock regardless of whether a change in keypad status has occurred. Effectively, therefore, the clock, two latches, and comparing circuitry of the keypad scanner consume power even during periods of inactivity of the keypad. Lacking keypad detectors that do not require power except when a change in keypad status occurs is a shortcoming and deficiency of the prior art.
Another disadvantage of prior art keypad scanners is their complexity. In order to operate, such keypad scanners must store an old key value into a first latch, read a new key value into a second latch, compare the new latched value with the old latched value, indicate to the computer if a difference exists between the two latched values, and then store in the first latch the new latched value if that value is different than the existing old latched value. This method of operation is highly complex and cannot be implemented inexpensively. Therefore, it would be advantageous to have a new, simpler, keypad scanner.
For the foregoing reasons, there is a need for a keypad scanner which requires less power in operation and which is less complex than prior art scanners.